(a) Field of the Invention
The present invention relates to an eyepiece and, more particularly, to an eyepiece comprising a graded refractive index lens (hereinafter referred to as a GRIN lens).
(b) Description of the Prior Art
Generally, eyepieces should be arranged that aberrations thereof are corrected favourably and, at the same time, the eye relief thereof is long. This is because, if the eye relief is short, it is inconvenient, for example, for a person who wears spectacles because he cannot put his eye to the position of the eye point. Besides, if spherical aberration of the image of the exit pupil of the objective, i.e., so-called spherical aberration of the pupil, is not corrected satisfactorily favourably, the field will be partially eclipsed at the time of observation. If curvature of field is large, the quality of image will become unfavourable toward the marginal portion of the field, and the image will become blurred.
For an eyepiece, it is difficult to make the eye relief long and, at the same time, to correct said aberrations favorably. To solve the above-mentioned problem, it is necessary to make the lens diameters large or to arrange that the lens system is composed of a large number of lenses. As a result, the cost of production becomes high and the lens system becomes very large. For correction of curvature of field, it is often arranged to correct aberrations of an eyepiece in combination with an objective instead of correcting aberrations of an eyepiece itself independently or to make the meridional image surface and sagittal image surface symmetrical with each other by arranging that one of them becomes positive and the other becomes negative so as to thereby make the apparent image surface flat.
Generally, it is known to use an aspherical lens or GRIN lens in addition to spherical homogeneous lenses as an element constituting a lens system so as to thereby increase the aberration correcting capacity. However, there exist only few known examples of eyepieces in which an aspherical lens or GRIN lens is used, for example, the eyepiece shown on pages 407 through 412 in No. 3, Volume 22 of Applied Optics issued on Feb. 1, 1983.
The eyepiece shown in the above-mentioned paper comprises a GRIN lens formed that the refractive index thereof is graded in the direction of the optical axis.
As described so far, most of known eyepieces are composed of homogeneous lenses only and most of them are arranged to comprise a large number of lenses. Therefore, it is difficult to assemble them and the cost of production is high. Moreover, when it is attempted to satisfactorily favourably correct curvature of field of an eyepiece itself independently, the number of lenses constituting the lens system becomes still larger and the lens diameters also become still larger. Besides, it is very difficult to ensure the eye relief about 0.8 time of the focal length which is the practically useful eye relief.
In case of the afore-mentioned known eyepiece comprising a GRIN lens formed that the refractive index thereof is graded in the direction of the optical axis, curvature of field and astigmatism are large, the eye relief is not satisfactorily long and, therefore, they cause problems in practical use.